1. Field of the Invention
The present invention relates to a solid state imaging device of a charge transfer type and particularly to a two-dimensional solid state imaging device having distinctive features in a scanning mechanism for readout of signals.
2. Description of the Prior Art
Generally, a solid state imaging device comprises photodetectors and a scanning mechanism on a semiconductor material such as silicon, so that imaging of light from a visual range to an infrared range can be made if the photodetectors are suitably selected. As compared with a conventional imaging device structured by electron tubes, a solid state imaging device has advantages in that it has a small size and a light weight with high reliability and that the portions to be adjusted at the time of manufacturing and assembling an imaging apparatus are considerably decreased. For this reason, solid state imaging devices have attracted special interest in a wide range of technical fields.
Solid state imaging devices are roughly classified, according to the scanning systems, into two types: an XY address type and a charge transfer type. Conventionally, devices of the XY address type are typically represented as devices using MOS switches and devices of the charge transfer type are typically represented as devices using CCDs (charge coupled devices).
However, the former devices using MOS switches have a problem in that they cannot be used for detection of a very small signal requiring a high S/N ratio. In such a device, spike noise is generated when a MOS switch turns on for reading of a signal. This spike noise is mixed in a detected signal to lower the S/N ratio of the detected signal. Furthermore, the generated spike noise differs dependently on the rows for reading and as a result, a noise called a fixed pattern noise is generated, which further lowers the S/N ratio of the detected signal.
On the other hand, in the latter devices using CCD, particularly in a device of an interline transfer system widely utilized recently since photodetectors can be selected in an arbitrary manner as in the previously stated MOS system, the area of the CCD portion for transfer is preferably designed to be as small as possible. This is because in this device, where CCDs for transfer are disposed between the lines of the photodetectors, it is desired to enlarge the effective area of the photodetectors in order to improve the imaging performance. However, it is known that the charge transfer capacity of CCD is proportional to the storage gate area for one row of CCDs assuming that the CCDs have the same structure. Accordingly, if the area of the CCD portion for transfer is made small, there is a problem that the maximum value of the charge to be transferred has to be limited on the other side.
For example, in "Platinum Silicide Schottky-Barrier IR-CCD Image Sensors" by M. Kimata et al. in JJAP, Vol. 21 (1982) Suppl.21-1, it is indicated that in conventional image sensors of the interline transfer system, a dynamic range is limited by the charge transfer capacity of the vertical CCDs.
These problems become serious particularly in such devices as infrared rays solid state imaging devices in which a small signal in a large background is to be detected.